Alkali metal chloride brines, such as sodium chloride and potassium chloride, may be electrolyzed to yield the alkali metal hydroxide, sodium hydroxide or potassium hydroxide, and chlorine. This process may be carried out in a diaphragm cell having an acidic anolyte and a basic catholyte, the anolyte and catholyte being separated by a permeable barrier.
The anodic reactions are: EQU Cl.sup.- .fwdarw. Cl + e.sup.- EQU 2Cl .fwdarw. Cl.sub.2
And the cathodic reactions are: EQU H.sup.+ + e.sup.- .fwdarw. H EQU 2h .fwdarw. h.sub.2
Typically the anolyte liquor is acidified brine having a pH of from about 2.5 to about 4.5 and the catholyte liquor is an aqueous solution of alkali metal hydroxide and alkali metal chloride. In a caustic soda diaphragm cell, the catholyte liquor contains from about 7 to 12 weight percent of sodium hydroxide and from about 10 to 15 weight percent of sodium chloride.
Typically in diaphragm cells, the anode has been either a graphite anode or an electrocatalytically coated valve metal anode. Valve metals are those metals that form a passivating, inert film upon exposure to acidic media under anodic conditions. Typical of the valve metals are titanium, tantalum, tungsten, zirconium, and hafnium.